After a long negotiation, I have concluded that it is better to start the MSE University project. Currently a bachelor program in pure mathematics is offered (theoretical physics and phd programs would be offered in future). To people who are interested in participating this project as students or tutors, please go to Registration for the MSE University and inform their enrollments.

Anonymous

7:48 AM

@MathematicsAminPhysics I guess this is an informal program (?) and not a officially recognized one

@DonThomasReyes It seems to me that the force you want is going to be $\frac{20 \text{kg}\cdot g \cdot \sin 30^\circ - 80\text{N}}{ \cos 30^\circ}$. Think about breaking both gravity and your horizontal force into a component that's parallel to the slope and a component that's perpendicular to the slope, and ignore the component that's perpendicular.

I'm posting in meta for the first time so please don't mind some culturally outrageous thing that I would end up doing here and inform me about my peculiarities. So I have this question on stack exchange physics, about the fermi level of materials
and I have not received any assistance on it as o...

I have an actual degree, not a PhD, from an actual university. I worked very hard for it, for a number of years. It's really important that this is not devalued by a proliferation of people claiming they have degrees, when they really don't.

5

user84215

This project is not suitable for those people whose main concerns are earning degree at all.

Are partial derivatives of tensors pseudotensor, I do recall back in the undergrad GR course whenever we tried to transform it into another coordinate, it does not just having a change of sign plus the jacobian matrices that showed it indeed transforms like a tensor?

@0celóñe7 No, I am not talking about the covariant derivative, "$\partial_\mu T^{\mu\nu}$" seemed to be the ordinary partial derivative of a tensor that is formally resembling a divergence, That cannot even be a pseudotensor?

@ACuriousMind But you should know that there are sufficiently bad coordinate systems, like ones where the metric decays as $1/r+1/r^\alpha$, where "invariant" quantities defined by such pseudotensors are in fact not invariant

there are some physicist shenanigans with integration by parts that fail

@0celóñe7 True. Still, I think it would be good to explicitly state that the divergence that's meant in Slereah's answer is a covariant divergence. You know how bad physicists are at this, why make it even harder for them? :P

Wald does write things like $\nabla_a V^b=\partial_a V^b+\Gamma^b{}_{ac}V^c$, which is horrendous

@ACuriousMind In abstract indices you denote the argument of a tensor (seen as a multilinear map) by an index. So if $\omega$ is a 1-form and $v$ a vector, then $\omega(v)=\omega_av^a$, etc.

You can use the flexibility of indices without having to define coordinate systems

That being said, often you want to work with Christoffel symbols, etc., and those are not tensors

And this being a notation, you can't really "prove" anything about it, but it's certainly against the spirit to write something like $\Gamma^a{}_{bc}$.

@ACuriousMind Now Straumann is also dishonest. He doesn't use special symbols for abstract indices. He does everything with indices in one style, then says "interpret the indices as abstract when applicable"

@MathematicsAminPhysics It'll be better if you don't call those "degrees". You might just say that the MSE University you are designing will offer "courses" (like MOOCs do) by voluntary teachers in chat rooms. You can set course timing as events. Anyone who is interested can register themselves there. However, since it is a virtual chat room we can't do anything if the teachers fail to turn up at the right time.

If you take a diagram representing Compton scattering then you have a photon and electron coming in and a photon and an electron going out. If you rotate the diagram 90º, i.e. swap the time and spaces axes, then you have another diagram with an electron and poitron coming in and two photons going out.

"You can rotate an vertex into eight different orientations, each describing a completely different physical process. Included are electron-positron creations and annihilations, which contain the essence of Einstein's mass-energy relation ." Interesting. Got to read about this

@Mostafaııııııııııııııııııııııı It's not Shankar's fault. Almost no physicist discusses these issue unless they come up - otherwise everyone just pretends one is doing linear algebra. Not saying that's the optimal state of things, but that's how it is

@JohnRennie I haven't read this well, but it seems to discuss the rotation of Feynman diagrams.

Anonymous

You could check out the book. But well, I don't think I know enough to help you with this. My knowledge is limited to pop-sci Feynman diagrams. I hope to take up a serious particle physics course someday in the near future.

OK so i want to learn about quantum mechanics and GR. I know single variable calculus and classical physics. I know newtonian mechanics and a modest amount of electromagnetism and optics. Can anyone recommend me path to achieve my goal stated above. thx

What this is likely referring to is the different "channels" one gets from diagrams that look as if they are rotated, but which should be thought of as different assignment of "in/out" to the external lines

Anonymous

2:49 PM

@Черенки I'm in the same situation as you. Start with the "for dummies" books and start learning Linear Algebra side-by-side (at least).

@ACuriousMind well, OK, you can take the rotation to mean a reassignment of the in and out channels. Anyhow the book @Blue linked says that yes rotating Feynman diagrams (in the sense I mean) doe always result in a diagram that makes physical sense.